A wireless local network area (WLAN) supports an access point (AP) of a distributed system (DS) and a basic service set (BBS) formed of a plurality of stations (STA) which are not an access point (AP).
The access point (AP) can communicate with a new protocol wireless station and a legacy protocol wireless station. The access point (AP) may be defined as another terms such as a base station (BS), a node B, or a sector. The access point (AP) may access an Ethernet hub or a switching board. The Ethernet hub may access a router that transmits data packets to a MODEM.
There are wireless stations (STAs) in a coverage area of one access point. Such a wireless station (STA) is a final user access device in a WLAN system. That is, a wireless station (STA) is a transceiver that converts wireless signals to digital signals routable to communication devices and accesses to access points that receive data packets and distributes the received data packets to other devices or networks. Such a wireless station may be embodied in various forms such as universal serial bus (USB), a personal computer memory card international association (PCMCIA) card, a USB dongle, a network interface card (NIC) adaptor connected to a device, and an integral wireless module. Hereinafter, an access point (AP) and a wireless station (STA) are referred as a station.
IEEE 802.11 defines a protocol standard related to a wireless local area network (WLAN). A medium access control (MAC) protocol defined in IEEE 802.11 uses a frame structure shown in FIG. 1 in order to exchange information between wireless stations.
Referring to FIG. 1, a typical MAC frame structure includes a frame control information field, a medium occupancy time information field, a receiving station address field, a transmission station address field, a sequence control information field, a QoS control information field, a frame body field, and a frame check sequence (FCS) field.
The frame control information field includes a protocol field for recording a protocol version such as IEEE 802.11 MAC version, a type and subtype field for identifying a type of a frame in use, and additional fields for storing various parameters to control a frame, such as a ToDS field, a FromDS field, an additional fragment field, a retry field, a power control field, an additional data field, a WEP field, and a sequence field.
The medium occupancy time information field is used as one of a frame for network allocation vector (NAV) setup, a frame transmitted during a contention free period (CFP), and a PS-inspection frame.
The sequence control information field is used to dump a fragmentation/reassemble frame or an overlapped frame. The sequence control information includes a 4-bits fragmentation number field and a 12-bit sequence number field.
The frame body field is a typical data field. The frame body field includes a payload. A frame check sequence (FCS) is used for integrity check of a frame received from a predetermined station.
As described above, a MAC frame essentially includes the frame control information field, the medium occupancy time information field, a receiving address (RA) field, and a frame check sequence (FCS) field. The other fields may be excluded or included according to a type of a frame for saving wireless resources. For example, a QoS control information field may be excluded in case of a data frame not using QoS. In case of an IEEE 802.11n dedicated frame, a high throughput (HT) control information field is additionally included for a HT function as well as the QoS field.
Some types of frames used in a WLAN require a response from a station receiving the frame. For example, frames that require a response are a request to send (RTS) frame, a block ACK request (BAR) frame, a data frame, and various request frames such as a Probe request frame, an Authentication request frame, and an Association request frame. Response frames for the response request frames are a clear to send (CTS) frame for the RTS frame, a block ACK (BA) frame for the BAR frame, an ACK or BA frame for the data frame, and a Probe response frame, an Authentication response frame, and an Association response frame for other request frames.
According to IEEE 802.11, a response frame for a response request frame is transmitted right after a short inter frame space (SIFS). Referring to FIG. 2, it is required to send a CTS frame for a RTS frame, an ACK frame for a data frame, a BA frame for a BAR frame, a BA frame for an Aggregated data frame after a short time interval such as SIFS after receiving corresponding response request frames.
Lately, users for a WLAN have been abruptly increased. There have been many studies made to increase data throughput provided from one BSS. For example, many studies have been made for developing a Very High Throughput (VHT) WLAN system.
The VHT WLAN system has been advanced to provide more than giga-bits of throughput by adapting a Multi-user Multiple Input and Multiple Output (MU-MIMO) technology and a multi frequency channel technology. Recently, a system using 4×4 MIMO and a channel bandwidth of more than 80 MHz has been tested to provide more than 1 Gbps throughput.
The MU-MIMO technology and the multi frequency channel technology enable one station to exchange frames with a plurality of stations through a plurality of communication paths. That is, one station can transmit data to a plurality of stations at the same time. Accordingly, the throughput of a basic service set (BSS) can be abruptly increased.
IEEE 802.11 recommends transmitting control frames to be receivable at all stations included in a corresponding BBS in order to prevent collision between frames on a wireless medium. It is even required to transmit control frames to be receivable at all stations although a plurality of communication paths are used in the MU-MIMO environment. For this, the control frame is transmitted without a MU-MIMO technology applied in a MU-MIMO environment. Further, the control frame is transmitted through a common channel commonly used by all stations in a multi frequency channel environment. Such a control frame includes a RTS frame, a CTS frame, an ACK frame, a BAR frame, a BA frame, and various Poll frames.
In case of using a plurality of communication paths, stations receiving a control frame may correspond to communication paths in one-to-one manner. Accordingly, a plurality of control frames can be transmitted through a plurality of communication paths at the same time by transmitting each control frame through a corresponding communication path. However, an overhead rate for transmitting/receiving control frames is increased and an advantage of using a plurality of communication paths is degraded.